KEYBOARDCONTROL
When the keyboard
is
not being read by the microprocessor, the Keyboard READISCAN control line
(PAO)
is
at
logic 0. All the outputs of the shift register are
at
logic 0.
If no key on the keyboard
is
pushed, all the inputs to the &input NAND gate (U13)are
at
logic 1 level. When
a
key
is
pushed, the
column containing that key will he grounded. The output of U13 goes to logic 1 and C7 (in the
debounce circuit starts to discharge. When the voltage across C7 reaches approximately +0.7
V
above ground, the debounce circuit will trigger the interrupt input on the PIA (U11, pin 18) indicating
that
a
key
is
being pushed.
-:
-CLEAR DISPLAY COUNTER
SELF-SCAN
:
:
SE LF-SCAN
-:
i-
DISPLAY MEMORY UPDATE
-:-
DISPLAY
SCANIUFI~RE (PA41
-
CLEARILOAD
CONTROL (PA5)+,
+t
CLOCK (PAl)
-f
:
DATA (PBO
-
PB7)
+'
i
+t
MEMORY
WRITE
(PA61
Figure 11 1
a.
Memory Update Mode Sequence
Scans by ArtekMedia © 2007
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